1. Field of the Invention
The present invention relates to a negative electrode carbon material for a lithium ion secondary battery and to a manufacturing method thereof. More specifically, the present invention relates to a negative electrode carbon material for a lithium ion secondary battery which is manufactured from a rice starch portion as a raw material and has an excellent effect on reducing the raw material cost, and relates to a manufacturing method thereof.
In the present invention, the term “rice starch portion” indicates a starch portion derived from rice which is called “middle-grade white bran” or “high-grade white bran” and an albumen portion containing a large amount of starch particles.
2. Description of the Related Art
JP-A-2001-266,850 discloses a technology in which rice bran is used as a raw material for a negative electrode carbon material for a lithium ion secondary battery. In this invention, the lees of rice bran obtained after oil is extracted from the rice bran, that is, the pericarp and testa in the rice bran, commonly called “red bran”, are used as a raw material and mixed with a thermosetting resin such as a phenolic resin, and the mixture is calcinated to thereby manufacture a carbon material for a negative electrode.
However, in the invention disclosed by this publication, as described in examples, the step for extracting oil from rice bran is indispensable. Without it, a large amount of volatile matter other than water floats at the time of calcinating, adheres to the inside wall of a furnace, and generates a bad smell, thus making the method impractical. In addition, the maintenance of an electric furnace after calcinating is necessary and the cost for this is indispensable. Further, a thermosetting resin such as a phenolic resin which is used in combination with rice bran is more expensive than rice bran. As a result, the merit of reducing the cost by using rice bran as one of the vegetable residues cannot be enjoyed.
Unhulled rice is the seed of a rice plant, and unpolished rice is obtained by removing the hull of this unhulled rice. The unpolished rice is composed of the pericarp, testa, embryo, and albumen, and the albumen is composed of an outer aleurone layer and an inner layer of a starch storing tissue. The pericarp and the testa are also called “red bran”. As for the mass of each tissue of the unpolished rice, from the exterior side, the red bran accounts for 5 to 7%, the embryo accounts for 2 to 3%, and the albumen accounts for 90 to 93% of the total.
The surface layer of the unpolished rice having a polished rice percentage L of 100 to 91% may be called “red bran layer”, the exterior portion of the albumen having a polished rice percentage L of 91 to 81% may be called “sapio layer”, and the interior portion of the albumen having a polished rice percentage L of 81 to 66% may be called “white bran layer”.
In general, in the process for polishing unpolished rice, the degree of polishing rice can be expressed by polished rice percentage L. The polished rice percentage L herein refers to the percentage of the mass of polished rice to the mass of unpolished rice as shown by the following expression 1.Polished rice percentage L (%)=(mass of polished rice÷mass of unpolished rice)×100  (1)
The polishing percentage refers to the percentage of the mass of grounds to the mass of unpolished rice as expressed by the following expression 2.Polishing percentage (%)=(mass of grounds÷mass of unpolished rice)×100  (2)
Further, the mass of unpolished rice is the total of the mass of polished rice and the mass of grounds as expressed by the following expression 3.Mass of polished rice+mass of grounds=mass of unpolished rice  (3)
Therefore, the following relational expression (4) is always established for the degree of polishing rice.Polished rice percentage L (%)+polishing percentage (%)=100 (%)  (4)
The polished rice percentage L of rice to be eaten as cooked rice is generally about 92 to 90%. Polished rice having a polished rice percentage L of 70 to 50% is generally used for the brewing of sake, and the quality of sake is generally improved by reducing the polished rice percentage L. A portion having a polished rice percentage L of 90 to 70% (polishing percentage of 10 to 30%) is called “middle-grade white bran”, and a portion having a polished rice percentage L of 70 to 50% (polishing percentage of 30 to 50%) is called “high-grade white bran”.
As for the above rice bran, the middle-grade white bran and the high-grade white bran are used as fodder and raw materials for confectionery. The development of new applications is desired for the effective use of the rice bran.